Add a variant of closest_points_first that takes a predicate

Part of CleverRaven#75945. The variant will be used in pathfinding to find the nearest
ramp or stairs without having to enumerate all points.

* Make find_point_closest_first generic over point and tripoint.
* Extract common code between closest_points_first variants

src/point.cpp

@@ -124,14 +124,19 @@ std::vector<tripoint> closest_points_first( const tripoint &center, int max_dist
 
 std::vector<tripoint> closest_points_first( const tripoint &center, int min_dist, int max_dist )
 {
-    const std::vector<point> points = closest_points_first( center.xy(), min_dist, max_dist );
+    std::optional<int> n = rectangle_size( min_dist, max_dist );
+
+    if( n == std::nullopt ) {
+        return {};
+    }
 
     std::vector<tripoint> result;
-    result.reserve( points.size() );
+    result.reserve( *n );
 
-    for( const point &p : points ) {
-        result.emplace_back( p, center.z );
-    }
+    find_point_closest_first( center, min_dist, max_dist, [&result]( const tripoint & p ) {
+        result.push_back( p );
+        return false;
+    } );
 
     return result;
 }
@@ -143,42 +148,26 @@ std::vector<point> closest_points_first( const point &center, int max_dist )
 
 std::vector<point> closest_points_first( const point &center, int min_dist, int max_dist )
 {
-    min_dist = std::max( min_dist, 0 );
-    max_dist = std::max( max_dist, 0 );
+    std::optional<int> n = rectangle_size( min_dist, max_dist );
 
-    if( min_dist > max_dist ) {
+    if( n == std::nullopt ) {
         return {};
     }
 
-    const int min_edge = min_dist * 2 + 1;
-    const int max_edge = max_dist * 2 + 1;
-
-    const int n = max_edge * max_edge - ( min_edge - 2 ) * ( min_edge - 2 );
-    const bool is_center_included = min_dist == 0;
-
     std::vector<point> result;
-    result.reserve( n + ( is_center_included ? 1 : 0 ) );
-
-    if( is_center_included ) {
-        result.push_back( center );
-    }
+    result.reserve( *n );
 
-    int x_init = std::max( min_dist, 1 );
-    point p( x_init, 1 - x_init );
-
-    point d( point_east );
-
-    for( int i = 0; i < n; i++ ) {
-        result.push_back( center + p );
-
-        if( p.x == p.y || ( p.x < 0 && p.x == -p.y ) || ( p.x > 0 && p.x == 1 - p.y ) ) {
-            std::swap( d.x, d.y );
-            d.x = -d.x;
-        }
-
-        p.x += d.x;
-        p.y += d.y;
-    }
+    find_point_closest_first( center, min_dist, max_dist, [&result]( const point & p ) {
+        result.push_back( p );
+        return false;
+    } );
 
     return result;
 }
+
+template <typename PredicateFn, typename Point>
+std::optional<Point> find_point_closest_first( const Point &center, int max_dist,
+        PredicateFn &&fn )
+{
+    return find_point_closest_first( center, 0, max_dist, fn );
+};

src/point.h

@@ -8,6 +8,7 @@
 #include <cstdint>
 #include <cstdlib>
 #include <functional>
+#include <optional>
 #include <ostream>
 #include <vector>
 
@@ -290,6 +291,32 @@ std::vector<tripoint> closest_points_first( const tripoint &center, int min_dist
 std::vector<point> closest_points_first( const point &center, int max_dist );
 std::vector<point> closest_points_first( const point &center, int min_dist, int max_dist );
 
+template <typename PredicateFn, typename Point>
+std::optional<Point> find_point_closest_first( const Point &center, int min_dist, int max_dist,
+        PredicateFn &&fn );
+
+template <typename PredicateFn, typename Point>
+std::optional<Point> find_point_closest_first( const Point &center, int max_dist,
+        PredicateFn &&fn );
+
+
+// Calculate the number of tiles in a square from min_dist to max_dist about an arbitrary centre.
+inline std::optional<int> rectangle_size( int min_dist, int max_dist )
+{
+    min_dist = std::max( min_dist, 0 );
+    max_dist = std::max( max_dist, 0 );
+
+    if( min_dist > max_dist ) {
+        return std::nullopt;
+    }
+
+    const int min_edge = min_dist * 2 + 1;
+    const int max_edge = max_dist * 2 + 1;
+
+    const int n = max_edge * max_edge - ( min_edge - 2 ) * ( min_edge - 2 ) + ( min_dist == 0 ? 1 : 0 );
+    return n;
+};
+
 inline constexpr tripoint tripoint_min { INT_MIN, INT_MIN, INT_MIN };
 inline constexpr tripoint tripoint_max{ INT_MAX, INT_MAX, INT_MAX };
 
@@ -380,4 +407,48 @@ inline const std::array<tripoint, 8> eight_horizontal_neighbors = { {
     }
 };
 
+template <typename PredicateFn, typename Point>
+std::optional<Point> find_point_closest_first( const Point &center, int min_dist, int max_dist,
+        PredicateFn &&predicate_fn )
+{
+    std::optional<int> n = rectangle_size( min_dist, max_dist );
+
+    if( n == std::nullopt ) {
+        return {};
+    }
+
+    const bool is_center_included = min_dist == 0;
+
+    if( is_center_included ) {
+        if( predicate_fn( center ) ) {
+            return center;
+        }
+    }
+
+    int x_init = std::max( min_dist, 1 );
+    Point p;
+    p.x = x_init;
+    p.y = 1 - x_init;
+
+    Point d;
+    d.x += 1;
+
+    for( int i = 0; i < *n; i++ ) {
+        const Point next = center + p;
+        if( predicate_fn( next ) ) {
+            return next;
+        }
+
+        if( p.x == p.y || ( p.x < 0 && p.x == -p.y ) || ( p.x > 0 && p.x == 1 - p.y ) ) {
+            std::swap( d.x, d.y );
+            d.x = -d.x;
+        }
+
+        p.x += d.x;
+        p.y += d.y;
+    }
+
+    return std::nullopt;
+}
+
 #endif // CATA_SRC_POINT_H